I don't doubt for a second that there were players who enjoyed certain aspects of XI that others consider flawed.
But there's a factor in that 500,000 that needs to be considered -- brand loyalty.
Peaking as the eighth most subscribers at that time certainly isn't anything to scoff at. But I'm curious, and I know we'll never know, how many of those 500,000 stuck it out because the game was called Final Fantasy.
Would they ever have gotten close to those numbers without Square's flagship monicker on the box? Especially back when it was a PS2 only title that cost a $99 USD investment (game and hard drive) before ever even logging on.
I think this is a good disclaimer for any "official data" regarding MMO subscriptions:
Found hereCooking the Stats
It is worth pointing out that there is no way to obtain irrefutable accurate information on subscribers. Only the companies responsible for the games have access to the statistics and their interpretation of an active account may be different from yours. The majority count anyone who is paying for a subscription, anyone in a free trial period or anyone who has been active on an account within the last month. They make no distinction for users with multiple accounts so they will be counted separately. Clearly the figures will not be representative of the number of users you can expect to find online within a game world at any given time – that number will be much lower.
Also, I know citing Wiki is generally taboo, but this comes specifically cited from Edge magazine in 2006 on the FFXI page:
So I think it's important to take that 500,000 with a grain of salt, as it comes from the yearly Vana'diel census and in no way can be relied upon as representative of sustained and active subscriptions.As of 2006, between 200,000 and 300,000 active players logged in per day, and the game was the dominant MMORPG in Japan.[4]
It's merely an approximation of accounts that had been active at some point in that year.
It's also worth noting that XI was plagued with RMT. So much so that they were able to wreck the game's economy twice. But those RMT also count toward the subscriptions.
But to the point of the OP: like most of XI, CoP was an acquired taste. It was ruined for many not by it's design, but by its community. XI's subscriber base, however large, was a very dedicated group of people.
Many players would blow through content as quickly as possible to be able to return to their regular end game schedule. Meaning players who fell behind or were new to the game were stuck.
CoP was great when it was fresh and the games limited player base was going at it willingly, but it failed the test of time. Many players finished battles vowing "never again."
The story still remains one of the best SE has crafted. But the "magic" of CoP cannot be reproduced, as it was something different for everyone.
tl;dr: There's no doubt XI was a financial success. There's also no doubt that those sustained subscribers found something wonderful in their own experience. However, to use numbers produced by a company to promote their product to try and justify a very specific aspect of a service that has been running for 10 years now is flawed. Some people enjoyed level caps, some dealt with it in the promise of something else they might enjoy.
And all of that is irrelevant to XIV as the developers need to let the game stand on its own legs without making it "like XI" or "not like "XI." They now better than anyone what worked and what didn't.